Polyoxymethylene (POM) resin has recently been used as a molding material for various end products (such as gears, reels, etc.) but the molded articles manufactured from this resin are not fully satisfactory in impact strength, thus setting many attempts going to improve POM in this respect.
However, for chemical structural reasons, POM resin has had no partner resin that is sufficiently compatible with it. Furthermore, POM resin is not thermally stable and, therefore, not suited for high-temperature blending.
A variety of core-shell polymers have been proposed for incorporation in many thermoplastic resins by the melt-blending technique for the purpose of, in the main, improving the impact strength of the final resin. Among them, a core-shell polymer comprising a rubbery polymer core and a glassy polymer shell has the advantage that its dispersibility is less sensitive to variations in melt-blending conditions and hence it can be uniformly dispersed with high reproducibility. These core-shell polymers are used as an impact modifiers for polycarbonate, poly(butylene terephthalate), polyamide, polyphenyleneoxide and their mixture, polymer alloy.
However, the prior art core-shell polymers have the disadvantage that the core-shell polymer contains the ingredient which promotes thermal decompsition of POM, due to drawbacks in the methods employed for their synthesis. Therefore, they can hardly be evenly blended with POM resin. Furthermore, even if they could be blended with POM resin more or less successfully, the resulting compositions are poor in thermal stability.
European Patent Laid-open Publication No. 115,373 discloses a POM resin composition containing a rubbery elastomer obtainable on emulsion polymerization of an C.sub.1-8 alkyl acrylate but as the special blending conditions are indicated therein, a fully stable POM resin composition is hard to come by under the ordinary blending conditions. Moreover, the composition is not improved in its thermal stability because of no consideration at the emulsion polymerization.
U.S. Pat. No. 4,713,414 discloses a POM resin composition containing core-shell polymers and reactive titanate. But this core-shell polymer also contains impurities decomposing POM resin. Therefore, a stable POM resin composition cannot be produced. A comparative example between core-shell polymers of this invention and of U.S. Pat. No. 4,713,414 and EP-A-115,373 is disclosed as a comparative example 1 in this application.
As a POM resin composition assuring an improved impact strength, the composition proposed in U.S. Pat. No. 4,804,716 is known. While it is a POM resin composition capable of forming a thermoplastic IPN (interpenetrating polymer network) with a polyurethane elastomer, assurance of sufficiently high impact strength calls for the use of a fairly large amount of polyurethane elastomer which should detract considerably from the modulus of the molded article, let alone the disadvantages of inadequate thermal stability and fluidity.
U.S. Pat. No. 4,639,488 discloses a POM resin composition containing a rubbery elastomer obtainable by emulsion polymerization of butadiene but this composition is also lacking in thermal stability.
U.S. Pat. No. 3,749,755 discloses a POM resin composition containing a rubbery grafted elastomer. But this composition is lacking in thermal stability and the rubbery grafted elastomer is not described as a core-shell polymer.
Japanese Patent Examined No. 15331/1984 discloses a method for producing a thermoplastic resin like acrylonitrile-acrylate-styrene (AAS resin) using emulsion polymerization technique improved on impact strength. This is, however, copolymer not a blend mixture.
European Patent Laid-open Publication No. 305,273 discloses a method of preparation for latex using non-ionic surfactant. This latex, however, is used to toughen a vinyl polymer, and is not a core-shell polymer.
Polymer blend composed of crystaline polymer has the defect that it has low strength and elongation on the weld line. POM is a highly crystaline polymer. Therefore, the POM composition incorporated with polyurethane elastomer in order to improve impact strength shows much lower strength and elongation on the weld line than pure POM.
Moreover, POM does not have good weatherability among engineering plastics. The POM composition incorporated in particular with polyurethane elastomer shows much inferior weatherability.
The present inventors have made intensive investigations to improve impact strength, thermal stability, strength and elongation on the weld line, and weatherbility of POM compositions. From the investigation, they have found that the POM composition comprising a certain core-shell polymer shows much higher impact strength and better weatherability than the pure POM. The POM composition also has improved thermal stability, and strength and elongation on the weld line.